CH. xviii.] THE ARTERIES. 213 



The Arteries. 



Distribution. The arterial system begins at the left ventricle 

 in a single large trunk, the aorta, which almost immediately after 

 its origin gives off in the thorax three large branches for the 

 supply of the head, neck, and upper extremities ; it then traverses 

 the thorax and abdomen, giving off branches, some large and 

 some small, for the supply of the various organs and tissues 

 it passes on its way. In the abdomen it divides into two 

 chief branches, for the supply of the lower extremities The 

 arterial branches wherever given off divide and subdivide, until 





Fig. 215. Transverse section through a large branch of the inferior mesenteric artery of a 

 pig. f, endothelial membrane ; i, tunica elastica interna, no subendothelial layer is 

 seen ; m, muscular tunica media, containing only a few wary elastic fibres ; . e, 

 tunica elastica externa, dividing the media from the connective-tissue adventitia, a. 

 (Klein and Noble Smith.) X 350. 



the calibre of each subdivision becomes very minute, and these 

 minute vessels lead into capillaries. Arteries are, as a rule, 

 placed in situations protected from pressure and other dangers, 

 and are, with few exceptions, straight in their course, and fre- 

 quently communicate (anastomose or inosculate) with other arteries. 

 The branches are usually given off at an acute angle, and the 

 sum of the sectional areas of the branches of an artery generally 

 exceeds that of the parent trunk ; and as the distance from the 

 origin is increased, the area of the combined branches is increased 

 also. After death, arteries are usually found dilated (not col- 

 lapsed as the veins are) and empty, and it was to this fact that 

 their name (aprrjpia, the windpipe) was given them, as the ancients 



